The invention relates to seafloor electromagnetic surveying for resistive and/or conductive bodies, for example for oil and other hydrocarbon reserves or subterranean salt bodies.
FIG. 1 schematically shows a surface vessel 14 undertaking controlled source electromagnetic (CSEM) surveying of a subterranean strata configuration using standard techniques [1]. The subterranean strata configuration in this example includes an overburden layer 8, an underburden layer 9 and a hydrocarbon reservoir 12. The surface vessel 14 floats on the surface 2 of a body of water, in this case seawater 4 of depth h meters. A submersible vehicle 19 carrying a source in the form of a horizontal electric dipole HED transmitter 22 is attached to the surface vessel 14 by an umbilical cable 16. This provides an electrical and mechanical connection between the submersible vehicle 19 and the surface vessel 14. The HED transmitter is supplied with a drive current so that it broadcasts an HED electromagnetic (EM) signal into the seawater 4. The HED transmitter is positioned a height z′ (typically around 50 meters) above the seafloor 6. The EM signals comprise transverse electric (TE) and transverse magnetic (TM) mode components.
One or more remote receivers 25 are located on the seafloor 6. Each of the receivers 25 include an instrument package 26, a detector 24, a floatation device 28 and a ballast weight (not shown). The detector 24 comprises an orthogonal pair of horizontal electric dipole detectors and an orthogonal pair of horizontal magnetic field detectors positioned a height z above the seafloor 6. Typically the detectors will lie on the seafloor so that z is in effect zero. The horizontal electric dipole detectors are sensitive to horizontal components of the electric fields induced by the HED transmitter in the vicinity of the receiver 25, and produce electric field detector signals therefrom. The horizontal magnetic field detectors are sensitive to horizontal components of the magnetic fields, for example the magnetic flux density, induced by the HED transmitter in the vicinity of the receiver 25, and produce magnetic field detector signals therefrom. The instrument package 26 records the detector signals for later analysis. Examples of suitable receivers are described by Constable [8] and U.S. Pat. No. 5,770,945 [9].
The HED transmitter 22 broadcasts EM signals that propagate outwards both into the overlying water column 4 and downwards into the seafloor 6 and the underlying strata 8, 9, 12. At practical frequencies for this method and given the typical resistivity of the respective media 4, 8, 9, 12, propagation occurs by diffusion of electromagnetic fields. The rate of decay in amplitude and the phase shift of the signal are controlled both by geometric spreading and by skin depth effects. Because in general the underlying strata 8, 9, 12 are more resistive than the seawater 4, skin depths in the underlying strata 8, 9, 12 are longer. As a result, electromagnetic fields measured by a receiver located at a suitable horizontal separation are dominated by those components of the transmitted EM signal which have propagated downwards through the seafloor 6, along within the underlying strata 8, 9, 12, and back up to the detector 24 rather than directly through the seawater 4.
A sub-surface structure which includes a hydrocarbon reservoir, such as the one shown in FIG. 1, gives rise to a measurable increase in the horizontal electric field component amplitudes measured at the receiver relative to a sub-surface structure having only water-bearing sediments. This is because hydrocarbon reservoirs have relatively high resistivities (typically 100 Ωm) compared to other subterranean strata (typically 1 Ωm) and so the EM signals are less attenuated. It is this enhancement in horizontal electric field amplitudes which has been used as a basis for detecting hydrocarbon reservoirs [1].
It is important when surveying for hydrocarbon reservoirs to carefully consider the orientation of the current flows induced by a transmitted EM signal. The response of seawater and subterranean strata (which will typically comprise planar horizontal layers) to EM signals is generally very different for TE mode components of the transmitted signal, which excite predominantly horizontal current flows, and TM mode components, which excite significant components of vertical current flow.
For TE mode components, the coupling between the layers comprising the subterranean strata is largely inductive. This means the presence of thin resistive layers (which are indicative of hydrocarbon reservoirs) does not significantly affect the EM fields detected at the surface as the large scale current flow pattern is not affected by the thin layer. On the other hand, for TM mode components, the coupling between layers includes a significant galvanic component (i.e. due to the direct transfer of charge between layers). For the TM mode even a thin resistive layer strongly affects the EM fields detected at the receiver since the large scale current flow pattern is interrupted by the resistive layer. It is known therefore that a significant component of the TM mode is required to satisfactorily perform an EM survey in the field of oil exploration.
However, sole reliance on the sensitivity of the TM mode components to the presence of a thin resistive layer can lead to ambiguities. The effects on detected EM fields arising from the presence of a thin resistive layer can be indistinguishable from the effects arising from other realistic large scale subterranean strata configurations. In order to resolve these ambiguities it is known to determine the response of the subterranean strata to both TM mode components (i.e. inductively coupled) and TE mode components (i.e. galvanically coupled) [1]. The TE mode is most sensitive to large scale subterranean structures, whereas the TM mode is more sensitive to thin resistive layers.
The HED transmitter 22 shown in FIG. 1 simultaneously generates both TE and TM mode components with the relative contribution of each mode to the signal at the receiver depending on the HED source-receiver orientation. At receiver locations which are broadside to the HED transmitter axis, the TE mode dominates the response. At receiver locations which are inline with the HED transmitter axis, the TM mode is stronger (although the TE mode is still present) [1, 2, 3, 4]. The response at receiver locations in both the inline and broadside configurations is governed by a combination of the TE and TM mode components, and these tend to work in opposition.
At inline receiver locations for a one-dimensional layered subterranean strata the electric fields induced at the receiver will be radial (i.e. parallel to a line joining the source to the receiver) while at broadside receiver locations they will be azimuthal (i.e. perpendicular to a line joining the source to the receiver). For in-between locations the direction of the induced electric fields will depend on the relative coupling between the transmitter and detector for the TE and TM modes, which will depend on the subterranean strata's resistivity structure, for example whether it contains a hydrocarbon layer. For this reason, with known surveying techniques it is important to measure the orientation of the detector so that the direction of the induced electric fields is known. However, it can be difficult to do this accurately which can lead to a significant source of error when interpreting data.
To determine the differing responses of subterranean strata to the TE and the TM modes it is known to rely on the geometric splitting of the modes, i.e. to collect electric field amplitude data for different source-receiver alignments. This approach provides complementary horizontal electric field amplitude data sets which are differently sensitive to the TE and TM mode components of the transmitted EM signals. During analysis, these complementary data sets are combined to reveal differences between the TE mode and TM mode coupling between the transmitter and the detector. These differences are indicative of the presence or not of a subterranean hydrocarbon reservoir.
A problem with the above described survey and analysis techniques is that they do not generally provide good results for surveys made in shallow waters. This is due to the presence of an ‘airwave’ component in the EM fields induced by the HED transmitter at the receiver. This airwave component is due to EM signals from the HED transmitter which interact with the air. Since air is non-conducting and hence causes little attenuation, the airwave component can dominate the fields at the receiver. The airwave component is principally due to the TE mode components. This is because the TE mode components are efficiently inductively coupled across the seawater-to-air interface. The TM mode components, on the other hand, do not couple well across this boundary and consequently do not contribute significantly to the airwave component. Because it has not interacted with the subterranean strata, the airwave component contains little information about subterranean resistivity. Accordingly, if the airwave contributes a significant component to the EM fields induced by the HED transmitter at the receiver, the sensitivity of the technique to subterranean resistivity structures, such as hydrocarbon reservoirs, is greatly reduced. The path of an example airwave component is schematically shown in FIG. 1 by a dotted line labeled AW. The magnitude of the airwave component is reduced as a function of separation between source and receiver only by geometric spreading. However, the airwave component is strongly attenuated by its passage through the conducting seawater. This means that in relatively deep water (large h) the airwave component is not very significant at the receiver and as such does not present a major problem. However in shallow water (small h), the airwave component does not pass through as much seawater and thus makes a larger contribution to the EM fields induced by the HED transmitter at the receiver. This contribution becomes greater still at increasing source-receiver horizontal separations. This is because (other than due to geometric spreading) the strength of the airwave component is relatively constant over a wide range of horizontal separations since any extra distance traveled by the airwave component is almost exclusively in the non-attenuating air. Other components of the EM fields induced by the HED at the receiver, such as those which pass through the subterranean strata and are of interest, travel through lower resistivity media and become increasing attenuated as they travel further. For these reasons, the airwave component tends to dominate the EM fields induced by the HED transmitter at the receiver for surveys made in shallow water, especially at large source-receiver horizontal separations.
The existence of the airwave as a dominant component of the detector signals limits the applicability of the above described surveying and analysis techniques. In shallow water the source-receiver separations over which the techniques can be applied is much reduced. This not only leads to a need to employ more receiver locations to adequately cover a given area, but also limits the depth beneath the seafloor to which the technique is sensitive. This can mean that a buried hydrocarbon reservoir in shallow water may not be detectable, even though the same reservoir would be detected in deeper water.
FIG. 2A is a graph schematically showing results of one-dimensional modeling of two example EM surveys of the kind shown in FIG. 1. One example corresponds to a survey performed in deep water (dotted line) and the other to a survey performed in shallow water (solid line). For each model survey the amplitude of a horizontal electric field component induced at the receiver in response to the HED EM transmitter is calculated per unit transmitter dipole moment and is plotted as a function of horizontal separation r between the HED transmitter and the receiver. For both model surveys, the subterranean strata configuration is a semi-infinite homogeneous half space of resistivity 1 Ωm. In the deep-water example, the subterranean strata configuration is located beneath an infinite extent of seawater. In the shallow-water example, it is located beneath a 500-meter depth of seawater. In both cases the seawater has resistivity 0.3 Ωm. The transmitter and receiver are separated along a line which runs through the axis of the HED transmitter (inline orientation). It is the component of detected electric field resolved along this direction which is plotted in FIG. 2A. The HED transmitter is driven by an alternating current (AC) drive signal at a frequency of 0.25 Hz.
The effect of the airwave component on the amplitude of EM fields induced by the HED transmitter at the receiver is clear. In the deep-water model survey, where there is no airwave component (because the water depth is infinite), the calculated electric field amplitude falls steadily with increasing horizontal separation. In the shallow-water model, however, where there is a strong airwave component, the rate of amplitude reduction sharply decreases at a source-receiver horizontal separation of about 5000 m.
FIG. 2B is a plot showing the ratio, p, of the two curves shown in FIG. 2A. The large deviations from unity seen in FIG. 2B highlight the difference between these curves. Since the only difference between the two model surveys is the presence or not of an airwave component, the ratio plotted in FIG. 2A effectively shows the relative strength of the airwave component in the detected signal compared to that which passes through the subterranean strata for the shallow-water model survey.
It is apparent from FIGS. 2A and 2B that at all but the very shortest horizontal separations (less that 1000 m) the detected electric field is significantly larger for the shallow-water model. For example, at a horizontal separation of 2500 m, the amplitude of the detected signal in the deep-water model survey is around 10−12 V/Am2. In the shallow-water model survey it is higher at around 10−11.5 V/Am2. This is due to the additional contribution of the airwave component. This level of increase shows that the airwave component has an amplitude more than double that of the component which has passed through the subterranean strata, and accordingly over two-thirds of the detector signal carries almost no information about the subterranean strata. At greater horizontal separations the airwave component dominates even more. In particular, it becomes especially pronounced beyond around 5000 m. At this point there is a break in the rate at which the detected electric field amplitude falls with increasing horizontal separation. At a horizontal separation of around 7000 m, the airwave component in the shallow-water example has an amplitude around twenty times greater than that of the signal which passes through the subterranean strata. This clearly imposes high requirements for the signal-to-noise ratio of data collected over these sorts of horizontal separations, as is generally the case when a small signal rides on a large background. It is apparent that the airwave significantly limits the usefulness of these surveying and analysis techniques in shallow water.
FIGS. 3A and 3B schematically show gray scale representations of the modeled sensitivity S of conventional CSEM surveying to resistivity structure beneath the seafloor for two different water depths. For FIG. 3A, the water depth h is infinite and for FIG. 3B it is 50 m. The model surveys are made at a transmission frequency of 0.25 Hz and the earth is assumed to be a uniform half-space of resistivity ρ=1 Ωm. Sensitivity is plotted as a function of depth d below seafloor and separation r between source and receiver. In deep water (FIG. 3A) the depth d below the seafloor to which the CSEM survey data are sensitive increases with source-receiver separation (as a basic rule of thumb the data are sensitive to structure down to a depth of around half the source-receiver separation). The effect of the airwave in shallow water (FIG. 3B) is to decrease the depth to which the data are sensitive. As a consequence deep target detection becomes impossible.
One proposed solution to the problem of the airwave dominating shallow-water surveys has been to rely on measurements of the vertical electric field components [10]. This is because vertical electric field components are less affected by the airwave. However, in practice there can be difficulties with this approach in practical surveys. This is because measurements of vertical electric field are significantly more prone to motion induced noise than more conventional measurements of horizontal components.